1. Field of the Invention
The present invention relates to a small, high performance optical system lens group for use mainly in a small photographing unit such as a digital camera, and more particularly to a combination lens utilizing a ceramic lens.
2. Description of the Related Art
In these days, various small photographing units such as cameras and image scanners have been in use which use photographing devices such as two-dimensional CCD's.
Then, in recent years, in optical equipment such as digital still cameras, small and high performance zoom lenses have been in use.
In a combination lens such as a zoom lens for optical equipment, compactness and high resolution are required.
Due to this, the applicant of this patent application has filed a patent application for an invention for specifying combination conditions for a small and high performance zoom lens for digital still cameras which is an inexpensive combination lens with a small number of lenses which makes up a zoom lens (Japanese Unexamined Patent Publication No. 2003-057542).
In addition, in these days, light-transmitting ceramics have been developed. These light-transmitting ceramics have higher refractive indices than those of optical glass, and their hardness and flexural strength exhibit higher values than those of optical glass. This has allowed to produce lenses which are suitable for the tendency of making optical systems small in size and thin in thickness.
As has been described above, while photographing units using two-dimensional CCD's are small in size and high in resolution and hence make high quality photographing units, due to reflection of image forming light on the surface of a CCD, part of light that is incident on a photographing device is reflected, and the resulting reflected light from the photographing device is incident on a lens provided in front of the photographing device from the rear thereof as stray light. Then, the stray light so incident on the lens is reflected on the surface of the lens so as to be incident again on the photographing device.
Then, in the small photographing unit, a distance between the photographing device and the lens is small, and there sometimes exists a risk that a ghost image is produced on an image by the stray light.
In addition, since light-transmitting ceramics have the higher refractive indices and greater hardness and flexural strength, when used for lenses, the light-transmitting ceramics become optimal for use in making an optical system in optical equipment smaller in size.
In the light-transmitting ceramics, however, while the refractive indices and reflectance of optical glass range roughly from 1.5 to 1.8 and 4 percents to 8 percents, respectively, the light-transmitting ceramics have greater refractive indices. In a case where a light-transmitting ceramic whose refractive index is 2.08 is used as a ceramic lens, the resulting reflectance becomes about 12 percents, which is extremely greater than the reflectance of the optical glass, and hence there has existed a problem that reflected light from the CCD is reflected by the lens and the resulting reflected light returns to the CCD to impose a great effect on the production of a ghost image.